Variable frequency radio frequency identification (RFID) tags

ABSTRACT

Various radio frequency identification (RFID) tags are described that dynamically vary their resonant frequency to reduce or eliminate the potential effects of electromagnetic “tag-to-tag” coupling. An RFID tag, for example, includes a main antenna tuned to a first resonant frequency, and switching circuitry that dynamically changes the resonant frequency of the main antenna. The switching circuitry may selectively couple electrical elements, such as capacitive elements, inductive elements, or combinations thereof, to vary the resonant frequency of the RFID tag. The RFID tag may include a sensing circuit that determines when to selectively couple the electrical element to the main antenna to adjust the resonant frequency of the main antenna.

TECHNICAL FIELD

The invention relates to radio frequency identification (RFID) systemsfor article management and, more specifically, to RFID tags.

BACKGROUND

Radio-Frequency Identification (RFID) technology has become widely usedin virtually every industry, including transportation, manufacturing,waste management, postal tracking, airline baggage reconciliation, andhighway toll management. A typical RFID system includes a plurality ofRFID tags, at least one RFID reader or detection system having anantenna for communication with the RFID tags, and a computing device tocontrol the RFID reader. The RFID reader includes a transmitter that mayprovide energy or information to the tags, and a receiver to receiveidentity and other information from the tags. The computing deviceprocesses the information obtained by the RFID reader.

In general, the information received from an RFID tag is specific to theparticular application, but often provides an identification for anarticle to which the tag is fixed. Exemplary articles includemanufactured items, books, files, animals or individuals, or virtuallyany other tangible article. Additional information may also be providedfor the article. The tag may be used during a manufacturing process, forexample, to indicate a paint color of an automobile chassis duringmanufacturing or other useful information.

The transmitter of the RFID reader outputs RF signals through theantenna to create an electromagnetic field that enables the tags toreturn an RF signal carrying the information. The transmitter makes useof an amplifier to drive the antenna with a modulated output signal.

A conventional tag may be an “active” tag that includes an internalpower source, or a “passive” tag that is energized by the field createdby the RFID reader antenna. Once energized, the tags communicate using apre-defined protocol, allowing the RFID reader to receive informationfrom one or more tags. The computing device serves as an informationmanagement system by receiving the information from the RFID reader andperforming some action, such as updating a database. In addition, thecomputing device may serve as a mechanism for programming data into thetags via the transmitter.

SUMMARY

In general, radio frequency identification (RFID) tags are describedthat automatically and dynamically vary their resonant frequency toreduce or eliminate the potential effects of electromagnetic“tag-to-tag” coupling. In some environments, the distance between thearticles is limited, and multiple articles may be simultaneously presentwithin the electromagnetic field produced by the reader antenna. As aresult, electromagnetic “coupling” may occur between some of the RFIDtags attached to the articles, which results in a shift of the resonantfrequency of some of the tags. This shift in resonance frequency maycompromise the ability of the RFID tags to communicate with the RFIDreader.

The techniques described herein automatically compensate for variationsin resonance frequency that may occur due to tag-to-tag coupling bychanging the resonant frequency at which the RFID tag backscatters radiofrequency energy. In this manner, the techniques may allow the RFID tagto maintain effective RFID communications even when experiencingsubstantial tag-to-tag electromagnetic coupling.

In one embodiment, an RFID tag comprises a main antenna tuned to a firstresonant frequency; and switching circuitry that dynamically changes theresonant frequency of the main antenna.

In another embodiment, a method comprises operating a main antenna of aradio frequency identification (RFID) tag at an associated resonatefrequency and dynamically changing the resonant frequency of the mainantenna.

In another embodiment, an RFID system comprises an RFID interrogationdevice, an RFID tag associated with an article, wherein theinterrogation device interrogates the RFID tag to obtain informationregarding the article, and a computing device to process the informationretrieved from the RFID interrogation device. The RFID tag includes amain antenna tuned to a first resonant frequency, an integrated circuitelectrically coupled to the main antenna that stores information of theassociated article, and switching circuitry that selectively couples oneor more elements to the main antenna to adjust the resonant frequency ofthe main antenna.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an RFID tag that automaticallyand dynamically varies its resonant frequency to reduce or eliminate thepotential effects of tag-to-tag coupling.

FIG. 2 is a circuit diagram illustrating one exemplary embodiment of anRFID tag capable of dynamically adjusting its resonant frequency.

FIG. 3 is a schematic diagram illustrating another embodiment of an RFIDtag capable of dynamically adjusting its resonant frequency.

FIG. 4 is a flow diagram illustrating exemplary operation of an RFIDtag, such as the RFID tag of FIG. 2, that auto-tunes its resonantfrequency to compensate for coupling with neighboring tags.

FIG. 5 is a flow diagram illustrating exemplary operation of an RFIDtag, such as the RFID tag of FIG. 3, which includes a capacitive switchto dynamically change its resonant frequency.

FIG. 6 is a flow diagram illustrating exemplary operation of an RFIDtag, such as the RFID tag of FIG. 3, which includes a MEMS switch todynamically change its resonant frequency.

FIG. 7 is a block diagram illustrating an exemplary RFID system that mayutilize RFID tags capable of varying their resonant frequencies to aidcommunication in environments wherein tag-to-tag coupling may occur.

DETAILED DESCRIPTION

In conventional radio-frequency identification (RFID) systems, RFID tagsmay interfere with one another when the RFID tags are placed in closeproximity to one another. It has been determined that electromagneticcoupling between such tags may result in a shift of the resonantfrequencies of the tags. This shifted resonant frequency may not providea given one of the RFID tags with a sufficient induced current to powerthe tag, thereby causing the RFID tag to be out of the detectablefrequency range of the interrogation device.

In general, the magnitude of the resulting frequency shift is dependentupon the distance between the RFID tags, the size of the tags, theamount of coplanar overlapping that occurs between the tags, and thetotal number of tags that overlap. Some exemplary measurements offrequency shift due to tag-to-tag coupling are shown in Table 1 belowwhen multiple tags where placed within a fixed proximity of each otherin an overlapping position. In Table 1, A is the height of the tag, B isthe width of the tag, N is the number of overlapping tags, X is thedistance between consecutive tags, and F is the resultant resonantfrequency of the tag. Although the exemplary measurements shown in Table1 are downward frequency shifts, the frequency shifts caused by tagcoupling may also be upward frequency shifts. As can be seen from Table1, electromagnetic coupling between ten overlapping tags can shift thefrequency at which the tags respond approximately 2 MHz, which mayresult in the inability for the reader to successfully communicate withthe tags, or significantly reduce the read range.

TABLE 1 A B X F (Inches) (Inches) N (Inches) (MHz) 2 3 1 — 13.56 2 3 2.75 13.2 2 3 10 .75 10.5 .5 1.5 1 — 13.56 .5 1.5 2 .375 13.4 .5 1.5 10.375 10.6

FIG. 1 is a block diagram illustrating an RFID tag 12 that automaticallyand dynamically varies its resonant frequency to reduce or eliminate thepotential effects of tag-to-tag coupling. In particular, RFID tag 12dynamically changes its resonant frequency to function in environmentsin which there is tag-to-tag coupling as well as environments where notag-to-tag coupling occurs. In other words, RFID tag 12 automaticallycompensates for shifts in resonance frequency that may occur due totag-to-tag coupling by changing the resonant frequency of RFID tag 12 sothat a sufficient current level is induced to power RFID tag 12, therebymaintaining effective RFID communications.

In the illustrated embodiment of FIG. 1, RFID tag 12 includes a mainantenna 14 that is electrically coupled to an integrated circuit 16,often referred to as an “RFID chip.” Main antenna 14 is tuned to aparticular frequency, which may be, for example, either the operatingfrequency of the RFID system, a lower frequency than the operatingfrequency, or a frequency higher than the operating frequency of theRFID system. Integrated circuit 16 often includes internal memory (notshown) for storing information related to the article to which RFID tag12 is affixed.

In operation, main antenna 14 receives RF energy from a source, andbackscatters RF energy in a manner well known in the art. It is thisbackscattered RF energy that provides a signal by which an interrogator,such as an RFID reader or detection system, obtains information fromRFID tag 12 and, more particularly, about an article with which RFID tag12 is associated. RFID tag 12 includes switching circuitry 18 thatautomatically changes the frequency at which main antenna 14 resonatesin order to compensate for situations in which the pertinent resonatingfrequency of the tag has been varied due to tag-to-tag coupling.Switching circuitry 18 may, for example, selectively switch in and outcircuit elements, such as capacitive or inductive elements, therebyadjusting the resonant frequency of main antenna 14 to alternate betweentwo or more resonant frequencies. For example, main antenna 14 may beinitially tuned to a frequency that is higher than the operatingfrequency of a surrounding RFID system in order to compensate forsituations where tag-to-tag coupling occurs. In some instancestag-to-tag coupling, therefore, reduces the pertinent resonancefrequency of main antenna 14, causing main antenna 14 to inducesufficient current to power RFID tag 12. When no tag coupling orinterference is present, switching circuitry 18 may automatically switcha capacitive element in parallel with main antenna 14 to reduce thefrequency at which main antenna 14 resonates, thus reducing thepertinent resonant frequency to the frequency of the surrounding system.Subsequently, if tag-to-tag coupling is present, switching circuitry 18may switch out the capacitive element, thus bringing the pertinentresonant frequency of main antenna 14 back up to the operating frequencyof the RFID system.

In some embodiments, switching circuitry 18 may adjust the resonantfrequency of main antenna 14 when RFID tag 12 is de-energized, i.e.,when main antenna 14 is no longer in an RF field. In other words,switching circuitry 18 may adjust the resonating frequency of mainantenna 14 for each power-up cycle. For example, switching circuitry 18may automatically switch in or out one or more circuit elements to causemain antenna 14 to resonate at a first frequency during a first power-upcycle and a second frequency during a second power-up cycle. In thisembodiment, RFID tag 12 may not need a mechanism for determining whichresonant frequency is most optimum for RFID tag 12. Alternatively,switching circuitry 18 may automatically adjust the resonant frequencyof main antenna 14 during a single power-up cycle. In this embodiment,RFID tag 12 determines whether main antenna 14 is resonating at anappropriate frequency and adjusts the resonant frequency of main antenna14 when the induced voltage is insufficient to power RFID tag 12.

In some embodiments, RFID tag 12 may include sensing circuitry, such asa sensing antenna 20, for use in determining whether switching circuitry18 should change the resonant frequency of main antenna 14. In otherwords, main antenna 14 is used for RFID communications, while sensingantenna 20 is used to determine whether switching circuitry 18 shouldchange the resonant frequency of main antenna 14. Switching circuitry 18automatically adjusts the resonant frequency of main antenna 14 based onwhich of main antenna 14 and sensing antenna 20 is operating at a moreoptimum frequency, i.e., closer to the system frequency. In this manner,sensing antenna 20 may be viewed as determining or sensing whether thereis tag-to-tag coupling, and switching circuitry 18 essentially adjuststhe resonant frequency of main antenna 14 based on whether tag-to-tagcoupling is detected, thereby allowing the main antenna to inducecurrent sufficient to power RFID tag 12. As a result, sensing antenna 20may be small relative to main antenna 14, and need not necessarily besufficiently sized to power RFID tag 12 itself.

For example, sensing antenna 20 may be tuned to the operating frequencyof the RFID system, and main antenna 14 may be tuned to a frequencyhigher than the operating frequency of the RFID system. In an RFIDsystem with an operating frequency of approximately 13.56 MHz, forexample, sensing antenna 20 may be tuned to approximately 13.56 MHz andmain antenna 14 may be tuned to approximately 20 MHz. Consequently,sensing antenna 20 induces sufficient current when: (1) RFID tag 12 isplaced in an RF field that is approximately the same frequency as thetuned frequency of the sensing antenna, and (2) insufficient tag-to-tagcoupling is experienced by the sensing antenna, i.e. the sensing antennais operating near to the system frequency. This induced current causesswitching circuitry 18 to switch a capacitive element in parallel withmain antenna 14, resulting in a reduction in the resonant frequency ofmain antenna 14. In this manner, the resonant frequency of main antenna14 is automatically lowered from its initial high frequency to theoperating frequency of the surrounding RFID system.

When other tags are in close proximity to RFID tag 12, sensing antenna20 experiences sufficient tag-to-tag coupling to reduce its resonantfrequency below the operating frequency of the RFID system. As a result,the current induced by sensing antenna 20 falls below the definedthreshold, causing switching circuitry 18 to switch out the capacitiveelement from main antenna 14, or if RFID tag 12 is not in the state withthe capacitive element switched in, then switching circuitry 18 does notact. The increase in resonant frequency of main antenna 14 caused byswitching out the capacitive element, however, is reduced to theoperating frequency of the RFID system due to the tag-to-tag coupling.In this manner, RFID tag 12 automatically adjusts the resonant frequencyof main antenna 14, and achieves communication regardless of whetherother RFID tags are present and cause tag-to-tag coupling.

Although RFID tag 12 of FIG. 1 includes only a single sensing antenna,RFID tag 12 may include multiple sensing antennas and higher-orderswitching circuitry in order to provide RFID tag 12 with the capabilityto resonate at two or more frequencies. For example, RFID tag 12 mayinclude two sensing antennas tuned to different frequencies. In thisembodiment, switching circuitry 18 optionally switches in and outmultiple circuit components to adjust the resonant frequency of mainantenna 14, thereby allowing RFID tag 12 to resonate at three differentfrequencies depending upon the resonating frequencies of the sensingantennas and the amount of tag coupling. Additional sensing antennas maybe utilized to increase the granularity by which switching circuitry 18accounts for tag-to-tag coupling and controls main antenna 14 toresonate at or near the RFID system operating frequency.

FIG. 2 is a circuit diagram illustrating one exemplary embodiment of anRFID tag 22 capable of dynamically adjusting its resonant frequency. Inthe illustrated embodiment, RFID tag 22 includes a main antenna 14, asensing antenna 20, and switching circuitry 18. Switching circuitry 18selectively switches a capacitive element 24 in and out of parallelconnection with main antenna 14 in order to change the resonantfrequency of main antenna 14 to compensate for tag-to-tag coupling withother RFID tags located in close proximity. In particular, switchingcircuitry 18 switches capacitive element 24 into and out of parallelconnection with main antenna 14 based on the amount of current inducedby sensing antenna 20.

As a result, switching circuitry 18 may be viewed as dynamicallycontrolling the resonant frequency of main antenna 14 based upon whethersensing antenna 20 is operating near the system operating frequency orhas experienced a reduction in resonant frequency due to tag-to-tagcoupling. In other words, switching circuitry 18 determines which ofmain antenna 14 and sensing antenna 20 is operating at the most optimumfrequency, i.e., a frequency closest to the operating frequency of theRFID system, and switches in and out capacitive element 24 in accordancewith the determination.

As shown in FIG. 2, main antenna 14 is represented as a capacitiveelement 26 and an inductive element 28. For example, capacitive element26 may represent the capacitance of an integrated circuit (not shown)that is electrically coupled to main antenna 14 as well as thecapacitive characteristics of conductive traces fabricated to form mainantenna 14. Inductive element 28 may represent the inductance of theconductive traces forming main antenna 14. Sensing antenna 20 alsoincludes a capacitive element 30 and an inductive element 32, which mayrepresent the capacitive and inductive characteristics of conductivetraces fabricated to form sensing antenna 20.

Switching circuitry 18 includes a diode 34, resistors 36A and 36Barranged to form a voltage divider 38, a transistor 40 and capacitiveelement 24. Diode 34 maintains the voltage resulting from the currentsinduced by antennas 14, 20 for application to transistor 40 by voltagedivider 38, thereby controlling the switching functionality of switchingcircuitry 18. Voltage divider 38 controls the switching of capacitiveelement 24 in and out of parallel connection with main antenna 14.

Resistors 36A and 36B of voltage divider 38 can be selected to controlthe threshold at which transistor 40 turns on. For example, when theresistance of resister 36A is larger than the resistance of resistor36B, transistor 40 will turn on with more induced current from sensingantenna 20. As described, transistor 40 is controlled by the voltageacross voltage divider 38, which is directly proportional to the currentinduced in sensing antenna 20. Transistor 40 thus acts like a switchthat switches capacitive element 24 into parallel connection with mainantenna 14 when activated, and switches out capacitive element 24 whendeactivated.

Specifically, when RFID tag 22 is placed in an RF field, e.g., when aninterrogation device is attempting to interrogate RFID tag 22,respective currents are induced in main antenna 14 and sensing antenna20. When the neighboring tags are far enough apart from RFID tag 22 thatinsufficient tag-to-tag coupling occurs, the current induced in sensingantenna 20 is stronger than the current induced in main antenna 14 dueto the fact that sensing antenna 20 is tuned to the operating frequencyof the RFID system, i.e., the frequency at which the interrogationdevice emits RF energy. The high current in sensing antenna 20 willraise the voltage across voltage divider 38 high enough to turntransistor 40 on, thereby switching capacitive element 24 into parallelconnection with main antenna 14. In this manner, transistor 40 acts as aswitch that is activated when the voltage of voltage divider 38 risesabove a controlled threshold set point. Placing capacitive element 24 inparallel with capacitive element 26 reduces the resonant frequency ofmain antenna 14, e.g., from 20 MHz down to 13.56 MHz, based on theamount of capacitance of capacitive element 24. Capacitive element 24may comprise any component with a storage capacitance, such as acapacitor, a diode, a transistor and the like.

However, when neighboring RFID tags are in close proximity to RFID tag22, the resulting tag-to-tag coupling will reduce the frequency at whichboth main antenna 14 and sensing antenna 20 resonate. When the tagcoupling reduces the resonant frequency of main antenna 14 far enough,main antenna 14 will be resonating at a frequency closer to the RFIDsystem operating frequency (e.g., 13.56 MHz) than sensing antenna 20.When this occurs, the current induced in main antenna 14 increases,causing the voltage drop of voltage divider 38 to fall below thethreshold set point. Therefore, transistor 40 is deactivated, andcapacitive element 24 is switched out of parallel connection with mainantenna 14. In this manner, the tag-to-tag coupling with neighboringtags will detune the frequency of main antenna 14 such that main antenna14 is operating near the frequency at which the RFID system isoperating, thus allowing RFID tag 22 to successfully communicate withthe RFID reader in environments where tag interference may otherwiseoccur.

Although the exemplary tag 22 illustrated in FIG. 2 operates at only twofrequencies, the variable frequency tag may be designed to operate atmore than two frequencies. For example, tag 22 may include four sensingantennas and a four-way switch that selects the antenna that isoperating most closely to the operating frequency of the RFID system.Furthermore, although in the example of FIG. 2 switching circuitry 18switches a capacitive element in parallel with main antenna 14,switching circuitry may switch a capacitive element in series with mainantenna 14, short out a capacitive element, or switch in an inductiveelement in either series or parallel with main antenna 14, orcombinations thereof, to change the resonant frequency of main antenna14. For example, main antenna 14 may be tuned to 13.56 MHz and sensingantenna 20 may be tuned to 20 MHz, and sensing circuitry 18 may shortout capacitive element 26 in order to increase the resonant frequency ofmain antenna 14 when the current induced in sensing antenna 20 exceedsthe desired threshold. Additionally, in some embodiments, switchingcircuitry 18 may measure which of the antennas 14, 20 is operating at amore optimum frequency using a current divider or some other circuitresponse measurement. The embodiment illustrated in FIG. 2 may beimplemented with multiple circuit elements, a single circuit element,within integrated circuit 16, or a combination thereof.

FIG. 3 is a schematic diagram illustrating another embodiment of an RFIDtag 44 capable of dynamically adjusting its resonant frequency. In theillustrated embodiment, RFID tag 44 includes a switch 46 that switchesbetween inductive elements 48A and 48B in order to selectively includeinductive elements 48A and 48B as additional loops for an antenna 42. Inthis manner, switch 46 is able to vary the frequency at which RFID tag44 resonates.

Inductive elements 48A and 48B comprise portions of conductive tracesfabricated to form additional loops for antenna 42. However, inductiveelements 48A and 48B could be any component with an inductance, orcompletely separate antennas. As illustrated in the example of FIG. 3,inductive element 48A is physically shorter than inductive element 48B.Therefore, RFID tag 44 resonates at a higher frequency when switch 46 ispositioned to contact inductive element 48A than when the switchcontacts inductive element 48B.

Switch 46 may comprise a low power microelectromechanical system (MEMS)switch, a capacitor switch, or other switching component. Switch 46 mayeither be designed to automatically switch between inductive element 48Aand inductive element 48B, e.g., during a single power-up cycle or onalternative power-up cycles. In one implementation, switch 46 comprisesa low-power MEMS switch that changes the resonant frequency of RFID tag44 at alternate power-up cycles. In this embodiment, MEMS switch 46 mayrequire less power to switch positions than is necessary for integratedcircuit 16 to function properly. Therefore, when RFID tag 44 receivesenough power to activate MEMS switch 46, but not enough to powerintegrated circuit 16, the MEMS switch changes position in an attempt todraw enough current/voltage from the RF energy to power integratedcircuit 16. MEMS switch 46 may, for example, change position after eachtime RFID tag 44 is powered down. In this manner, RFID tag 44 wouldalternate resonating at two different frequencies, e.g., 13.56 MHz and20 MHz, at every other power-up cycle.

An interrogation device, therefore, may attempt a first read of all theRFID tags at a location. During the first read, depending on theircurrent state, a first subset of the RFID tags will be configured toresonate at a system operating frequency, e.g., 13.56 MHz, and theremaining tags will be configured to resonate at the second frequency,e.g., 20 MHz. If no tag-to-tag coupling occurs, the interrogation devicecommunicates with the first subset during the first read cycle, and thenremoves and reapplies the electromagnetic field to cause the RFID tagsto change their resonant frequency. As a result, the first subset oftags will be configured to operate at the second frequency and theremaining tags will be configured to operate at the system frequency.Thus, the interrogation device will be able to communicate with the tagsin no more than two interrogation cycles.

If there is tag-to-tag coupling, however, during the first interrogationcycle the resonant frequency of the RFID tags may be reduced. Inparticular, some or all of the RFID tags configured to operate at thesystem operating frequency will be reduced below a frequency rangeresponsive to the interrogation device. Therefore, these RFID tagsexperiencing tag-to-tag coupling with neighboring tags may notsuccessfully communicate with the interrogation device. However, theRFID tags configured to resonate at the system operating frequency thatdo not experience tag-to-tag coupling will be able to successfullycommunicate with the interrogation device. In addition, the resonantfrequencies of some or all of the RFID tags configured to operate at thesecond frequency, e.g., 20 MHz will be similarly reduced closer to theoperating frequency, and will be able to communicate with theinterrogation device.

After reading all the detectable tags on the first read, theinterrogation device automatically removes the RF field and initiates asecond interrogation cycle, which causes the MEMS switches of the RFIDtags to change position. During the second interrogation cycle, all ofthe tags that were unable to communicate with the interrogation deviceon the first interrogation attempt will be able to on the second readdue to the changed inductance. Particularly, during the secondinterrogation cycle, the first subset of tags that were originallyconfigured to resonate at the system operating frequency will now beconfigured to resonate at the second operating frequency, e.g., 20 MHz.The portion of these tags for which communication during the firstinterrogation cycle was unsuccessful due to tag-to-tag coupling will nowbe detuned by the tag-to-tag coupling from the second resonate frequencyto a frequency closer to the system operating frequency and will achievesuccessful communication with the interrogation device. During thesecond read cycle the RFID tags that were originally configured toresonate at the second frequency will be configured to resonate at thesystem operating frequency. Those tags that were unable to communicateduring the first read cycle, i.e., those tags that were not experiencingtag-to-tag coupling, will now achieve successful communication with theinterrogation device. In this manner, in this configuration, theinterrogation device is able to read all of the RFID tags in two readcycles or less. For embodiments of RFID tag 44 that use more than twoadditional inductive elements, e.g., three or more inductive elements,additional interrogation cycles may be utilized to ensure that all ofthe RFID tags achieve successful communication regardless of tag-to-tagcoupling. These interrogation cycles can be merged and recorded by theinterrogation device to obtain an accurate log of articlessimultaneously present at the location.

In an alternate implementation, switch 46 selectively switchescapacitive elements in and out of antenna 42 to change the resonantfrequency of the antenna. Upon applying an RF field, RFID tag 44resonates at a first frequency, e.g., the operating frequency of theRFID system (13.56 MHz), while the capacitor begins to collect charge.As soon as the capacitor collects enough charge, i.e., when it has beenin the RF field for an extended period of time, switch 46 switches to a“charged” switch position. In the example illustrated in FIG. 3, thecharged switch position may be the position that contacts inductiveelement 48B. At the “charged” switch position, RFID tag 44 resonates ata second frequency that allows communication to be achieved whensufficient tag-to-tag coupling occurs, e.g., 20 MHz. After the capacitorloses enough energy to be unable to hold the capacitor in the “charged”switch position, e.g., after the interrogation cycle is terminated,switch 46 changes back to the “uncharged” switch position. In thismanner, switch 46 has a default switch position, e.g., initiallyresonating at 13.56 MHz. In this manner, RFID tag 44 automaticallyvaries its resonant frequency during a single power-up cycle to ensuresuccessful communication with the RF reader. In other words, in thisconfiguration, the interrogation device need not apply two separateinterrogation cycles.

Although in the example of FIG. 3 switch 46 of RFID tag 44 switchesbetween two traces of different lengths to change the inductance ofantenna 42, RFID tag 44 may use N different trace lengths and a 1 to Nswitch in order to allow N different frequency variations. Furthermore,switch 46 may be used to switch in other types of components, such ascapacitive components, inductive components, or combinations thereof, toautomatically adjust the resonant frequency of RFID tag 44.Additionally, RFID tag 44 may include sensing circuitry as describedwith respect to FIG. 2 to allow RFID tag 44 to select the switchposition based upon which of the switch positions causes antenna 42 tooperate at an optimum frequency, thereby auto-tuning RFID tag 44. Theembodiment of FIG. 3 may be implemented with multiple “tag elements,” ona single tag element, within an RFID chip, or a combination thereof.

FIG. 4 is a flow diagram illustrating exemplary operation of an RFIDtag, such as RFID tag 22 of FIG. 2, that auto-tunes its resonantfrequency to compensate for coupling with neighboring tags. Initially,RFID tag 22 is placed in an RF field (50). For example, RFID tag 22 mayreceive RF energy from an RFID detection device or reader antenna thatis attempting to retrieve information from the RFID tag. The RF fieldwithin which RFID tag 22 is placed induces a current in both mainantenna 14 and sensing antenna 20.

In general, switching circuitry 18 determines whether RFID tag 22 isexperiencing sufficient tag-to-tag coupling to detune main antenna 14,which is set to a resonant frequency higher than the operating frequencyof the RFID system, to resonate at an appropriate frequency, i.e., in afrequency range responsive to an interrogation device (54). For example,switching circuitry 18 may determine whether a current induced insensing antenna 20 is sufficiently large to raise it beyond a thresholdvalue. When switching circuitry 18 determines that insufficienttag-to-tag coupling is occurring, i.e., when the current induced insensing antenna 20 is larger than a threshold value, transistor 40 turnson (56) and switches a capacitive element 24 in parallel with mainantenna 14 (58). As described above, capacitive element 24 increases thecapacitance of main antenna 14, thereby reducing the resonant frequencyof main antenna 14 from the higher frequency, e.g., 20 MHz, to thesystem operating frequency, e.g., 13.56 MHz (62).

When switching circuitry 18 determines that sufficient tag-to-tagcoupling is occurring to detune main antenna 14 from the higher resonantfrequency to the system operating frequency, the switching circuitryturns off transistor 40 or transistor 40 remains off (60). As a result,main antenna 14 is configured to resonate at the higher frequency, butis detuned by the tag-to-tag coupling with neighboring tags in closeproximity (62). In this manner, RFID tag 22 auto-tunes itself to achieveresonance at the system operating frequency in order to compensate fortag-to-tag coupling using a transistor 40 that is controlled based onthe current induced by sensing antenna 20 relative to the currentinduced by main antenna 14. In other embodiments, other types of sensingand switching mechanisms may be used to auto-tune RFID tag 22.

FIG. 5 is a flow diagram illustrating exemplary operation of an RFIDtag, such as RFID tag 44 of FIG. 3, which includes a switch 46 (e.g., acapacitive switch in this example) to dynamically change the resonantfrequency of main antenna 14 without sensing whether main antenna 14 isresonating at an optimum frequency. Initially RFID tag 44 is placed inan RF field (64), and the tag is configured by default to resonate at afirst frequency, such as the operating frequency of the RFID system,e.g., 13.56 MHz (66). However, if tag coupling is present, the resonantfrequency will be reduced, possibly below a frequency range at which thetag is able to communicate with the interrogation device.

As a result of the electromagnetic field, the capacitive switch 46begins to accumulate charge (68). When the charge of capacitive switch46 collects enough charge, it switches to a “charged” switch position(70), and RFID tag 44 resonates at a second frequency (72), e.g., 20MHz. When tag-to-tag coupling between neighboring tags exists, antenna42 will be detuned from the higher resonant frequency to the loweroperating frequency as described above. In this manner, the resonantfrequency of RFID tag 44 varies during a single power-up cycle to ensuresuccessful communication with the interrogation device without requiringtwo separate interrogation cycles. After the capacitor loses enoughcharge, e.g., when the interrogation field is removed, capacitive switch46 is unable to hold the “charged” switch position, and the capacitiveswitch switches back to the “uncharged” switch position (74).

FIG. 6 is a flow diagram illustrating exemplary operation of an RFIDtag, such as RFID tag 44 of FIG. 3, which includes a MEMS switch todynamically change the resonant frequency of main antenna 14 withoutsensing whether main antenna 14 is resonating at an optimum frequency.In this example, switch 46 comprises a MEMS switch that dynamicallychanges the resonant frequency of main antenna 14 between power-upcycles as described above.

Initially, RFID tag 44 is placed in an RF field (76), and RFID tag 44 isconfigured to resonate at a first frequency, such as the operatingfrequency of the RFID system, e.g., 13.56 MHz (78). If tag-to-tagcoupling occurs, however, the resonant frequency of RFID tag 44 may bereduced below the frequency range responsive to the interrogationdevice, and the RFID tag may not successfully communicate with theinterrogation device. The RF field is subsequently removed (80), e.g.,after the interrogation devices reads all the RFID tags that respond tothe first interrogation cycle, causing MEMS switch 46 to changepositions (82). When a subsequent RF field is applied to RFID tag 44(84), e.g., during a subsequent interrogation cycle, the RFID tag isconfigured to resonate at a higher frequency, e.g., 20 MHz (86), whichwill be detuned when tag-to-tag coupling with neighboring tags exists.

FIG. 7 is a block diagram illustrating an exemplary RFID system 88 thatmay utilize RFID tags capable of varying their resonant frequencies toaid communication in environments wherein tag-to-tag coupling may occur.In the illustrated example of FIG. 7, RFID system 88 is used fordocument and file management. RFID system 88 may, for example, bedeployed within law offices, government agencies, or other facilitiesthat generate and store documents and files, such as business, criminal,and medical records.

In practice, these files may be positioned in a number of storage areas90, e.g., as shown in FIG. 7, on an open shelf 90A, a cabinet 90B, avertical file separator 90C, a cart 90D, a pad 90E or a similarlocation. Storage areas 90 may be provided at multiple locations withinan organization, as opposed to in a single file room. For example, astorage area 90 may be associated with a particular location, e.g., adocketing shelf, and thus may be referred to or considered to be“dedicated” shelves. As also described below, storage areas 90 could belocated near individual offices or other areas in, for example, ahospital or clinic, a law firm, an accounting firm, a brokerage house,or a bank.

In some embodiments, storage areas 90 may be “smart storage areas.” Theterm “smart storage area” is used generally to refer to a storage areafor a document or other item that is equipped with RFID interrogationcapability to aid in tracking and locating documents or files withinsystem 88. In particular, smart storage areas may include one or moreantennas that are used to read RFID tags associated with the itemsstored in the respective storage areas and communicate the informationread from the RFID tags to article management system 92 that provides acentral data store, e.g., within one or more databases of a relationaldatabase management system (RDBMS), for aggregation of the information.The information may include, for example, position information thatindicates where the file or document is located. File tracking system 92may be networked or otherwise coupled to one or more computers so thatindividuals at various locations can access data relative to thoseitems. For example, a user may use file tracking system 92 to retrievethe file location information from the data store, and report to theuser the last location at which the items were located within one of thestorage areas.

In an environment as illustrated in FIG. 7, the distance between thefiles or documents and, more particularly, the RFID tags associated withthose articles generally cannot be fixed to a minimum distance. Inaccordance with the techniques described herein, the RFID tags of system88 dynamically and automatically vary their resonant frequency to aidcommunication to an interrogation device, such as a smart storage area.As a result, tag-to-tag coupling may occur, yet the RFID tags mayachieve increased ability to reliably communicate with the RFIDinterrogation devices of system 88.

RFID system 88 operates within a frequency range of the electromagneticspectrum, such as 13.56 MHz, with an allowable frequency variance of+/−7 kHz, which is often used for Industrial, Scientific and Medical(ISM) applications. However, other frequencies may be used for RFIDapplications, and the invention is not so limited. For example, someRFID systems in large storage areas such as a warehouse may use an RFIDsystem that operates at approximately 900 MHz.

Various embodiments of the invention have been described. These andother embodiments are within the scope of the following claims.

1. A radio-frequency identification (RFID) tag comprising: a mainantenna tuned to a first resonant frequency; switching circuitry thatdynamically changes the resonant frequency of the main antenna; and asensing circuit to sense electromagnetic coupling with a neighboringRFID tag and activate the switching circuit in response to the sensedeletromagnetic coupling.
 2. The RFID tag of claim 1, further comprisinga capacitive element, wherein the switching circuitry selectivelyelectrically couples the capacitive element to the main antenna tochange the resonant frequency of the main antenna.
 3. The RFID tag ofclaim 2, wherein the switching circuitry couples the capacitive elementin parallel with the main antenna to reduce the resonant frequency ofthe main antenna.
 4. The RFID tag of claim 2, wherein the capacitiveelement comprises one of a capacitor, a diode, and a transistor.
 5. TheRFID tag of claim 1, further comprising an inductive element, whereinthe switching circuitry selectively electrically couples the inductiveelement to the main antenna to dynamically change the resonant frequencyof the main antenna.
 6. The RFID tag of claim 1, further comprising: afirst conductive trace of a first length; and a second conductive traceof a second length, wherein the first length is greater than the secondlength, and further wherein the switching circuitry selectively couplesthe first conductive trace or the second conductive trace to the mainantenna to change the resonant frequency of the main antenna.
 7. TheRFID tag of claim 1, wherein the switching circuitry comprises amicroelectromechanical system (MEMS) switch that selects differentelectrical elements to change the resonant frequency.
 8. The RFID tag ofclaim 1, wherein the switching circuitry comprises a capacitive switchthat changes the resonant frequency of the main antenna based upon astored charge.
 9. The RFID tag of claim 1, wherein the sensing circuitsenses an amount of electromagnetic coupling with the neighboring RFIDtag, and activates the switching circuit to selectively increase ordecrease the resonant frequency of the main antenna based on the sensedamount of electromagnetic coupling.
 10. The RFID tag of claim 1, whereinthe switching circuitry comprises a transistor that turns on when acurrent in the sensing circuit exceeds a threshold value.
 11. The RFIDtag of claim 1, wherein the switching circuitry further comprises afirst resistor and a second resistor arranged to realize a voltagedivider to regulate the threshold value at which the transistor turnson.
 12. The RFID tag of claim 1, further comprising a sensing antennatuned to a second resonant frequency, wherein the switching circuitrychanges the resonant frequency of the main antenna based on an amount ofcurrent induced within the sensing antenna.
 13. The RFID tag of claim12, wherein the main antenna and the sensing antenna are coplanar. 14.The RFID tag of claim 12, wherein the sensing antenna is tuned toapproximately 13.56 megahertz (MHz) and the main antenna is tuned toapproximately 20 MHz.
 15. The RFID tag of claim 1, wherein the switchingcircuitry automatically changes the resonant frequency of the mainantenna upon application or removal of a radio frequency field to theRFID tag.
 16. The RFID tag of claim 1, further comprising an RFIDintegrated circuit electrically coupled to the main antenna that storesinformation of an associated article and communicates the information toan RFID reader via the main antenna.
 17. A method comprising: operatinga main antenna of a radio frequency identification (RFID) tag at anassociated resonate frequency; sensing an amount of electromagneticcoupling between the RFID tag and a neighboring RFID tag; anddynamically changing the resonant frequency of the main antenna based onthe sensed amount of electromagnetic coupling.
 18. The method of claim17, wherein dynamically changing the resonant frequency comprisesselectively coupling a capacitive element to the main antenna toselectively increase or decrease the resonant frequency of the mainantenna.
 19. The method of claim 18, wherein the capacitive elementcomprises one of a capacitor, a diode, and a transistor.
 20. The methodof claim 17, wherein dynamically changing the resonant frequencycomprises selectively coupling an inductive element to the main antennato change the resonant frequency of the main antenna.
 21. The method ofclaim 17, wherein dynamically changing the resonant frequency comprisesselectively coupling a first conductive trace of a first length or asecond conductive trace of a second length to the main antenna to changethe resonant frequency of the main antenna.
 22. The method of claim 17,wherein the RFID tag includes a sensing antenna having a resonantfrequency different from the resonant frequency associated with the mainantenna, and wherein dynamically changing the resonant frequencycomprises dynamically changing the resonant frequency associated withthe main antenna when a current induced in the sensing antenna exceeds athreshold value.
 23. The method of claim 22, wherein the resonantfrequency of the sensing antenna is tuned to approximately 13.56megahertz (MHz) and the resonant frequency of the main antenna is tunedto approximately 20 MHz.
 24. The method of claim 17, wherein dynamicallychanging the resonant frequency comprises dynamically changing theresonant frequency of the main antenna upon application or removal of aradio frequency field to the RFID tag.
 25. A radio frequencyidentification (RFID) system comprising: an RFID interrogation device;an RFID tag associated with an article, wherein the interrogation deviceinterrogates the RFID tag to obtain information regarding the article;and a computing device to process the information retrieved from theRFID interrogation device, wherein the RFID tag includes a main antennatuned to a first resonant frequency, an integrated circuit electricallycoupled to the main antenna that stores information of the associatedarticle, switching circuitry that selectively couples one or moreelements to the main antenna to adjust the resonant frequency of themain antenna, and a sensing circuit to sense electromagnetic couplingwith a neighboring tag and activate the switching circuit in response tothe sensed electromagnetic coupling.
 26. The system of claim 25, whereinthe one or more elements includes a capacitive element, and theswitching circuitry selectively couples the capacitive element to themain antenna.
 27. The system of claim 26, wherein the switchingcircuitry selectively couples the capacitive element in parallel withthe main antenna to reduce the resonant frequency of the main antenna.28. The system of claim 26, wherein the capacitive element comprises oneof a capacitor, a diode, and a transistor.
 29. The system of claim 25,wherein the one or more elements includes an inductive element, and theswitching circuitry selectively couples the inductive element to themain antenna.
 30. The system of claim 25, wherein the one or moreelements includes a first conductive trace of a first length and asecond conductive trace of a second length, and the switching circuitryselectively couples the first conductive trace or the second conductivetrace to the main antenna.
 31. The system of claim 25, wherein theswitching circuitry comprises one of a microelectromechanical system(MEMS) switch and a capacitive switch.
 32. The system of claim 25,wherein the sensing circuit senses an amount of electromagnetic couplingwith a neighboring RFID tag, and activates the switching circuit toselectively couple the one or more elements to the main antenna based onthe sensed amount of electromagnetic coupling to selectively increase ordecrease the resonant frequency of the main antenna.
 33. The system ofclaim 32, wherein the main antenna and the sensing antenna are coplanar.34. The system of claim 32, wherein the sensing antenna is tuned toapproximately 13.56 megahertz (MHz) and the main antenna is tuned toapproximately 20 MHz.
 35. The system of claim 25, wherein the sensingcircuit comprises a sensing antenna tuned to a second resonant frequencydifferent from the first resonant frequency, and the switching circuitryselectively couples the one or more elements to the main antenna whenthe current in the sensing antenna exceeds a threshold value.
 36. Thesystem of claim 35, wherein the switching circuitry comprises atransistor that turns on when the current in the sensing antenna exceedsthe threshold value.
 37. The system of claim 36, wherein the switchingcircuitry further comprises a first resistor and a second resistorarranged to realize a voltage divider to regulate the threshold value atwhich the transistor turns on.
 38. The system of claim 25, wherein theswitching circuitry automatically changes the resonant frequency of themain antenna upon application or removal of a radio frequency field tothe RFID tag.